Frequency filter for electric currents



Jan. 5 1926. 1568,115

H. w. ELSASSER FREQUENCY FILTER F OR ELECTRIC CURRENTS Filed Feb. 26, 1921 3 Sheets-Sheet 1 vwmtoz flWZZswser W5 W W- VFK I FREQUENCY FILTER FOR ELECTRIC cuagmnrrs Filed Feb. 26, 1921 I5 Sheets-Sheet 2 Jan. 5 1 :32 6.

H. W. ELSASSER FREQUENCY FILTER FOR ELECTRIC CURRENTS Filed Feb. 26.,

1921 I 3 Sheets-Sheet 5 p in Patented Jan. 5, 1926 UNITED STATESfirn'rsu'riorrics.

may w. nnsnssnn, or NEW YORK, 1e. 1, Assronon romemo/m TELEPHONE AND TELEGRAPH COMPANY, n conronnmonor New Yon rnnounnoy FILTER non nnno'rnxo qunnnnrsi Application filed February 26, 1921. Serial No. 448,029.

To all whom it 'mag concern Be it known that I, HENRY W. ELSASSKR, a citizen of the United States, residing at New York, in the county of New York and 5 State of New York, have invented certain Improvements in Frequency Filters for Electric Currents, of which the following a s ecification. 1'

t is an object of my invei'ition to provide a new and improved wave filter for alternating currents of various frequencies. Another object of my invention is to provide an input termination for a filter of known fl type, so that the filter thus terminated shall over the free transmission range of frelquencies, or over the most essential partof much ran e. Still another object of my invention has relation to providing an improved balancing network for ocean ci blc operation.

In the the'anompanying drawings. I have disclosed limited number oi specific embodi ments of the inventionnnd I now proceed :to describe those embodiments, with the understanding-that the invention will be defined in the appended claims. I-

In the drawings, Figure}. is u diagram in connection with the explanation of the theory thereof; Fig. 7 is a diagram that will be referred to when occasion arises to discuss the mid-shunt characteristic impedance of a filter; or other recurrent network; Fig.

8 is a diagram by the aid of which llly lll'lr proved filter may be compared with other types with respect to its equality of substantialconstancy of impedance and Fig. 9 is a diagram for con'iparisori with Fig. l in conneotion with the explanation of the theory of the apparatus of Fig. l.

have a substantially constant lIIlPBdlHCE' following specification, taken with of awave filter, or other urtificialline, with 6 are diagrau'is tor compaiison with Fig. 1

As is known, there is a class of artificiel' lines, each of which consists of a network of recurrent sections as in Fig. 1, Where Z is a series impedance, Z is ashunt impedance,

end the combination of these two impedances is repeated successively in the'structure from left to right. The theory of such a network may be developed for an infinite number of sections extending from a given pair of terminals such as B and D in Fig. 1.

In practice, a moderate finite number of sections will behave almost likethe infinite network and the practical filter or other artificial line consists-of such a finite number of sections with suitable terminal modifications.

Stated hrieiiy generally,such a. network, functioning as a filter tinnsmits freely currents of a frequency lying Within a certain range and shunts out currents of a fre quencylying in another range.

At the input end the filter may be terininuted at any fractional point of'a. series olcmentor oft a shunt element. In Fig. l, the filter begins at the terminals B and D in mid-series termination, which means that the same. Having given the filter proper,

extending to the right terminals B and l) as shown in Fig. l, it will now call attention to my improved input terminal network. This consists ofthe various iinpedunces having the values Z /r, 21 /2, ZZ /2 and 2,, disposed and connected as shown in the die.- grani. The effect of this net interposed at the. input end of the filter is to make the impedanco oi the combination substantially constimt'over the free tra'ismitting range, or more nearlyconstont than it would he without the net. That this is the eiiect of the interposed network will (be demonstrated presently. l

A more specific illustration is given in Fig. 2, where the filter is of the low pass type,

meaning that it freely transmits all fre quencies up to Mei-min critical value of frequency, but shunts out currents of higher frequency. In the low pass filter, the series impedances Z, ure'sii'nply the impedances of the induction coils L and the shunt impedance; Z correspond to the reactances of loo the shunt condensers C The construction of the network will be apparent from Fig. 2 and it will be understood that the condensers of capacity 2C correspond to the impedances Z /2 of Fi 1.

The problem of balancing a long ocean cable is one of considerable importance, for such cables are always operated by the bridge method. which requires an artificial line whose impedance should be essentially the same as that of the cable for a considerable range of frequencies. Fig. 3 shows the usual bridge at one end of the cable G, balanced against the artificial line or network H. This consists of an artificial balancing network of known type designated H, connected through a few sections of the artificial line having my improved input termination as shown in the diagram, where the construction is so fully shown as not to require verbal exposition.

In Fig. 4, a somewhat different arrangement of the balancing network for the cable has been shown. The theory of these net- ,works will be understood after explanation of the general theory has been given.

As I have stated, an advantage of my improved filter and terminal network is that they have a nearly constant impedance over the tree transmitting range. To show this, I will compare their impedance over this range with the impedance of the same filter but with the common mid-series or midshunt termination, which has heretofore been employed. The mid-series termination is the one shown for the filter at the terminals Band D in'Fig. 1, as already explained. The mid-shunt termination is shown in Fig. 7 where it will be apparent that the filter begins with a shunt element of half the usual admittance, that is, with the impedance 2Z. the series elements all being equal and the remaining shunt elements all being equal but having the impedance value Z The mid-series characteristic impedance across the terminals B and D in Fig. 1 is, of course the same in the infinite filter as across any other mid-series points. Hence we have the following equation:

the solution of which gives Having obtained an expression for the value otthc mid-series characteristic impedance Z it will now be shown that we can substitu e for Fig. 1 ihe simpler finite diagram of *ig. For the star at A in Fig. 1, subz-stitutethe equivalent delta A in Fig. 5; comprising the three impedances OZ /Q, 5Z. ./-l and 5Z /2. These values are obtained by the well known star-delta transformation, as explained, for example, in paragraph 34; and context, 2 of the Standard Handbook for Electrical Engineers. 4th edition. published in 1915 by the McGraw-Hill Book Company.

Vith this explanation, mere inspection at the diagrams will show t e validity of the substitution of Fig. 5 for Fig. 1 in the problem of obtaining the value of Z. Fig. 6 represents a further stage where the two parallel impedances between the points E and i are combined ihto the single impedance .Z,Z /(10Z,+2Z,).

By combining the parallel branches between .l and F in Fig. 6, then between J and E, and representing the prodpct Z,Z. by Z,, and the quotient Z,/Z by 9 the result a is reached by ordinary procedure, that 2 Z 2 10+10g +2g+ /1+g /4 (12g+4g At this point I will digress briefly to dewhence velop a value of Z to be substituted therefor. Making use of the symbols Z and 1 Z 1 (4) that have just been defined, it'tollows from 1 2 Equation 1 that -19 In connection with Fig. 7, it will readily be seen that Adding Equations 3 and 4, the result is ob- Zo ml mlh) means Substituting this value of Z in Equation 2 gives Li-r.

o a rwa The foregoin Equation 5 is an expression for the impe ance Z of the combined filter and terminal network of Fig. 1, in convenlent form for demonstratlon of the substantial constancy of the value of Z.

In the applications that are of practical interest, the absolute value of g is always From Equation 3, it will beseen that as r departs from zero, Z increases from Z and from Equation4 it will be seen that at the same time Z decreases from Z,. For the particularcase of Fig. 2, these facts are shown by the diagram of Fig. 8. In this case, the abscissa: are frequency ratios instead of values of 1, but this will be readily appreciated when it is understood that for the low-pass filter of Fig. 2,

Thus it will be seen that not only in the particular case involved in Figs. 2 and-8, but

' in general, Z is more nearly constant (and equal to Z than is either Z, or Z For frequencies near the end of the free transmission range, opposite to that for which Z=Z the deviation evidently becomes considerable but it is a deviation that gives the filter as a whole an increasing impedance, so that its only effect is somewhat to hasten and blur the point of cut-off at the corresponding end of the frequency range.

The mid-series and mid-shunt terminations for which the impedances are represented by Z,,, and 71 are common filter terminations; it will be seen that compared with these terminations, my improved termination gives a more nearly constant imcdance over the free transmitting range of rcquencies.

' A helpful view of the improvement may he allordedby consideration of Fi 9, for which the structure is a parent. e conductor extending from to M being consid- Accordingly in Equation 5 credv to have zero resistance, the diagram evidently exhibits two filters in series, the first lying between the terminals N and KM and the second between K-M and P. The first of these two filters has mid-series termination, the second mid-shunt. Their combined impedance across the terminals N and P is Z +Z Replacing Z and Z, by Z /2 and Z /2 in Equations 1 and 4, the combined impedance of the network of Fig. 9 is found to be.

By the same course of rea- (ill somng presented heretofore in connection with Fig. 8, the impedance of the network of Fi .9 is more nearly constant than is the impedance of either. of its two component filters which are connected in series. Equation 6 amounts to a demonstration th'at'the resistanceless connector K-M may be omitted between points K and M. For

some purposes, it may be preferable to kees the connections the same as in Fig. 9 i stea of as in Fig. 1.

,VVhereas Fig. 9 represents the'two filters of respective mid-series and mid-shunt termination connected in series, Fig. 4 shows two such artificial lines connected in parallel. Here again the resultant im edance across the terminals of the com ination averages the impedances of the component elements and gives a more nearly constant value. v

I claim:

1. A wave filter of the recurrent section tvne and a network at the input end consisting of a midseries input network and a 1nidshunt input network connected in series between the input terminals and across the main body of. the said filter whereby the filter has animpedance value approximately the average of the values that it would have with each termination alone.

2. A filter of the recurrent section type having two input terminals and an input terminal network comprising two series 1mpedances connected respectively to the two terminals of the filter, and an impedance star with one ray resting connected with an intermediate tap in one of said series impedances and the other rays connected to the respective terminals of the other series impe ance.

' 3. A wave filter of the type having recurrent sections extendihg from a pair of input terminals, said filter being the equivalent of two filters with their input terminals connected in series, one of them having a midseries termination at the input end and the otherharing a midshunt tern'iination at the input end whereby' the impedance or they filter as a whole averages the impedances that would be obtained if the filter had either of said terminations alone.

4. A vave filter of the recurrent network type two input terminals'therefor, respective impedance elements connecting said liher with said terminals and a three terminal mesh with one terminal associated with one said impedance element and the 15 other two terminals associated with the other said impedance element.

5. A wave filter of the type having recurent sections and having" its impedance made substantially constant over, a considerable 20- I n testimony whereof, I have signed my name to this specification this18th day of blehruary 1921.

' HENRY W. ELSASSER. 

